Erectile dysfunction (ED) commonly affects the quality of life of men after treatment of prostate cancer. We conducted a placebo-controlled, crossover randomised trial to assess the efficacy and tolerability of sildenafil citrate in the treatment of ED developing after external beam radiation treatment (EBRT) of localized prostate cancer. Sixty-six patients who had developed ED following radiation treatment agreed to participate and were allocated to sildenafil or placebo to be taken prior to four sexual attempts. In the crossover period, subjects received the alternative tablet for a further four attempts. Allocation was centrally randomized, and researchers and patients were both blinded to the trial arm. Efficacy was assessed using the International Index of Erectile Function (IIEF) questionnaire and with a separate global efficacy question. Forty-three subjects completed the study. There was a significant increase in mean scores from baseline for all domains of the IIEF with sildenafil compared with placebo (P < 0.001). Affirmative response to the global efficacy question was more common after taking sildenafil compared with placebo. In approximately half of the patients, the improvement in the erectile function domain score corresponded to a moderate improvement in ED (e.g. success ‘sometimes’ to ‘most times’). Sildenafil was associated with mild flushing, nasal stuffiness or indigestion in 8–10% patients and moderate flushing in 10%. The current study adds to the evidence that phosphodiesterase inhibitors are an effective and well-tolerated treatment for ED after EBRT for prostate cancer.